1. Field of the Invention
This invention relates to an apparatus and method of printing a conductive heater grid design on plastic or glass glazing panels, such as those used as backlights in vehicles.
2. Related Technology
Plastic materials, such as polycarbonate (PC) and polymethylmethyacrylate (PMMA), are currently being used in the manufacturing of numerous automotive parts and components, such as B-pillars, headlamps, and sunroofs. Automotive rear window (backlight) systems represent an application for these plastic materials due to their many identified advantages, particularly in the areas of styling/design, weight savings, and safety/security. More specifically, plastic materials offer the automotive manufacturer the ability to reduce the complexity of the rear window assembly through the integration of functional components into the molded plastic system, as well as the ability to distinguish their vehicles by increasing overall design and shape complexity. Being lighter in weight than conventional glass backlight systems, their incorporation into the vehicle may facilitate both a lower center of gravity for the vehicle (and therefore better vehicle handling & safety) and improved fuel economy. Further, enhanced safety is realized, particularly in a roll-over accident because of a greater probability of the occupant or passenger being retained in a vehicle.
Although there are many advantages associated with implementing plastic windows, these windows are not without limitations that represent technical hurdles that must be addressed prior to wide-scale commercial utilization. Limitations relating to material properties include the stability of plastics during prolonged exposure to elevated temperatures and the limited ability of plastics to conduct heat. Regarding the latter, in order to be used as a backlight in a vehicle, the plastic material must be compatible with the use of a defroster or defogging system (hereafter just referred to as a “defroster”). For commercial acceptance, a plastic backlight must meet the performance criteria established for the defrosting or defogging of glass backlights.
The difference in material properties between glass and plastics becomes quite apparent when considering heat conduction. The thermal conductivity of glass (Tc=22.39×10−4 cal/cm-sec-° C.) is approximately 4-5 times greater than that exhibited by a typical plastic (e.g., Tc for polycarbonate=4.78×10−4 cal/cm-sec-° C.). Thus, a defroster designed to work effectively on a glass window may not necessarily be efficient at defrosting or defogging (hereafter just “defrosting” or “defrost”) a plastic window. The lower thermal conductivity of the plastic may limit the dissipation of heat from the heater grid lines across the surface of the plastic window. Thus, at a similar power output, a heater grid on a glass window may defrost the entire viewing area, while the same heater grid on a plastic window may only defrost those portions of the viewing area that are close to the grid lines.
A second difference between glass and plastics that must be overcome is related to the electrical conductivity exhibited by a printed heater grid. The thermal stability of glass, as demonstrated by a relatively high softening temperature (e.g., Tsoften>>1000° C.), allows for the sintering of a metallic paste on the surface of the glass window to yield a substantially inorganic frit or metallic wire. Since the softening temperature of glass is significantly greater than the glass transition temperature of a typical plastic resin (e.g., polycarbonate Tg=145° C.), a metallic paste cannot be sintered onto a plastic panel. Rather, it must be cured on the panel at a temperature lower than the Tg of the plastic resin.
A metallic paste typically consists of metallic particles dispersed in a polymeric resin that will bond to the surface of the plastic to which it is applied. The curing of the metallic paste provides a conductive polymer matrix having closely spaced metallic particles dispersed throughout a dielectric layer. The presence of the dielectric layer (e.g., polymer) between dispersed conductive particles leads to a reduction in the conductivity, or an increase in resistance, of the cured heater grid lines, as compared to dimensionally similar heater grid lines sintered onto a glass substrate. This difference in conductivity manifests itself in poor defrosting characteristics exhibited by the plastic window, as compared to the glass window.
With the above in mind, it is clear that controlling the quality of the heater grid printed onto the panel is important to maximizing the efficiency and effectiveness of any defroster used with that panel. Various parameters affect the quality of the printed heater grid and these parameters include any variances in the width, height and straightness of the grid lines. The more variances that exist in width and height, the greater the negative impact on the effectiveness of the defroster. This is a result of unequal resistances in various sections of the grid line and busbars resulting in unequal resistive heating in various sections of the defroster. With regard to straightness, this is mainly an aesthetic concern that becomes more of an issue because of the ability of plastic window assemblies to have greater design flexibility and curvature.
A defroster may be printed directly onto the surface inner or outer of a panel, or on the surface of a protective layer, using a conductive ink or paste and various methods known to those skilled in the art. Such methods include, but not limited to, screen-printing, ink jet printing and automatic dispensing. Automatic dispensing includes techniques known to those skilled in the art of adhesive application, such as drip & drag, streaming, and simple flow dispensing. In each of the above instances, the shape of the panel impacts the quality of the printed lines, i.e. screen printing becomes very difficult on non-planer panels, and the speed at which printing is done both the width and height of the grid lines. Slower speeds and higher flow for the ink or paste rates can result in wider and higher grid lines. Conversely, higher speeds and slower flow rates can result in slimmer and lower grid lines. With screen printing in particular, the height of the grid line is not readily variable.
From the above, it is seen that there is a need in the industry for an apparatus and method that can effectively control the quality and consistency with which grid lines are printed onto a panel.